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Why are there some sounds that you can continue making like “sssssss” or “ooooo” and sounds that you can only make once and can’t repeat like “k” or “d”?

2 Answers 2

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You can articulate any phoneme for as long as you are able, but (1) you may not be able to articulate some phonemes as long as others and (2) some phonemes are by themselves inaudible, and can only be detected via their effect on other sounds.

In the case of [o] or [s], you can put your face in the required configuration and you can breathe, sustaining [o] for about 15 seconds and [s] for about 45 seconds. These sounds can be heard because air is flowing continuously through the vocal tract (speech sound is modulation of airflow coming out of the vocal tract). You can form a [k] and hold it for about 4 minutes, but you won't hear anything, until you release the k and start making some other sound. The reason you don't hear anything is that the vocal folds (the source of voicing) are abducted and the pressure below and above the glottis equalizes almost instantly, and then you're just holding your breath until you are compelled to breath in. Pressure equalization in the case of [k] is because there is nowhere for the oral air to go, whereas with [o] and [s], air can escape the mouth, thus there can be continuous sound. [d] is closer to [k], except that the vocal folds are adducted, causing voicing (thus you can hear something), but because [d] still completely blocks the flow of air out of the mouth, pressure builds up in the oral cavity very quickly, and you can only maintain [d] for about 1 second (you get better mileage with [b], maybe twice as much time phonating, because you can let your cheeks balloon out, although we don't do that in actual speech).

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While the other answer is technically correct, here's one that might be more readily accessible.

Sounds are generally made and distinguished based on where you put your tongue (of course).

For some sounds, your tongue is relatively low and allows a constant stream of air to pass over it. This is the basic definition of a vowel: 'oo', 'ah', 'ee' ...

For some sounds, your tongue nearly closes the gap, but not quite. The air experiences some "friction" as it passes over. For example, 'sss', 'fff', 'thhh'... Because of that friction these are called fricatives.

For other sounds, your tongue completely closes the gap. The air is actually blocked, and the sound you hear is the air being released. These are called "stops" and include 'd' and 'k'.

Now, you can't release the air slowly when making these sounds. It's like striking a match. Either it's fast or your brain doesn't think it's a 'd' or a 'k'.


Interestingly, you can actually hold those sounds, in a way. The nose is a second air channel. Try holding 'nnn' for a while. You'll find that what you're doing is putting your tongue where you make 'd', and letting air pass through the nose. These "nasal" sounds are almost as loud as vowels.

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